Yellow fever is a viral disease that has caused large epidemics in Africa and the Americas. It can be recognized from historic texts stretching back 400 years. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The "yellow" in the name is explained by the jaundice that affects some patients. Although an effective vaccine has been available for 60 years, the number of people infected over the last two decades has increased and yellow fever is now a serious public health issue again. In mild cases, yellow fever causes fever, headache, nausea and vomiting. But yellow fever can become more serious, causing bleeding (hemorrhaging), heart, liver and kidney problems. Up to 50 percent of those with the more severe form of yellow fever die of the disease. Symptoms of Yellow Fever Many yellow fever infections are mild, but the disease can cause severe, life-threatening illness. Symptoms of severe infection are high fever, chills, headache, muscle aches, vomiting, and backache. After a brief recovery period, the infection can lead to shock, bleeding, and kidney and liver failure. Liver failure causes jaundice (yellowing of the skin and the whites of the eyes), which gives yellow fever its name. Chest congestion develops in many patients, and abdominal pain and discomfort are common. The fever becomes constant. Improvement occurs in the third and fourth week in those without complications. About 10% of patients have recurrent symptoms (relapse) after feeling better for one to two weeks. Relapses are actually more common in individuals treated with antibiotics. The illness may progress to liver and renal failure, and hemorrhagic symptoms and signs caused by thrombocytopenia and abnormal clotting and coagulation may occur. The case-fatality rate of yellow fever varies widely in different studies and may be different for Africa compared to South America, but is typically 20% or higher. Jaundice or other gross evidence of severe liver disease is associated with higher mortality rates. The fever pattern is biphasic and is called a dromedary pattern, reflecting the 3 phases of the illness described above. In the acute phase, fevers may be high, with relative bradycardia. The saddle occurs during the secondary stage, when temperatures normalize. In the intoxication phase, fevers recur. Infection ranges from asymptomatic (in 5 to 50% of cases) to a hemorrhagic fever with 50% mortality. Incubation lasts 3 to 6 days. Onset is sudden, with fever of 39 to 40 C, chills, headache, dizziness, and myalgias. The pulse, usually rapid initially, by the 2nd day becomes slow for the degree of fever (Faget's sign). The face is flushed and the eyes are injected. Nausea, vomiting, constipation, severe prostration, restlessness, and irritability are common. When a person becomes infected with the yellow fever virus, the virus begins to multiply. After three to six days, the symptoms of yellow fever can begin to appear. The period between becoming infected with virus and the appearance of yellow fever symptoms is called the incubation period. The virus remains silent in the body during an incubation period of three to six days. There are then two disease phases. While some infections have no symptoms whatsoever, the first, "acute", phase is normally characterized by fever, muscle pain (with prominent backache), headache, shivers, loss of appetite, nausea and/or vomiting. Often, the high fever is paradoxically associated with a slow pulse. After three to four days most patients improve and their symptoms disappear.
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